This invention relates to a parallel operation system for stabilized power sources used in industrial applications such as controlling microcomputers.
In general, when a design engineer constructs a power source system by parallel-connecting a plurality of power sources, two significant objects must be satisfied; specifically, reliability of the power supply and an increase in its capacity are desired.
FIG. 1 is a block diagram showing one example of a conventional power source system which is a so-callled "diode matching system".
In FIG. 1, reference numerals 1 and 2 designate power sources which are operated in a parallel mode, and reference characters D1 and D2 designate output matching diodes. These diodes are used to prevent the output current of one of the power sources from flowing into the other of the power sources when the output voltage of one of the power sources is greater than that of the other, respectively.
In the case where two power sources are operated in parallel as described above, the power source which has a greater output voltage (for example, source 1 and of FIG. 1) supplies substantially 100% of the load current. Under this condition, should the power source 2 be deactivated, the load is not affected because power source 1 still supplies output current to the load. On the other hand, should the power source 1 be deactivated, power source 2 starts supplying current to the load. Thus, in both cases, the load is constantly supplied with load current.
As is clear from the above description, the parallel power source diode matching system of the prior art is advantageous in that the number of components required is relatively small, and accordingly the arrangement is simple. However, the operation of the parallel power source diode matching system is disadvantageous for the following reasons:
(1) In practice, the difference between the output voltages of the parallel power sources will never become zero. Therefore, it is difficult to maintain load balance between the power sources; that is, the load current is always supplied by only one of the power sources. Accordingly, the temperature of the power source supplying the load current increases, such that the power source itself (and accordingly the power source system) is degraded in reliability. Since the reliability of the power source system depends upon the power source which supplies the load current, even if the number of power sources to be parallel-operated is increased, the reliability of the system is not improved.
(2) In the case where the parallel operation is carried out in order to increase the output capacity, the load balance is not sufficient to maintain both power sources in their conductive states. As a result, the load current is supplied by only one of the power sources, and it becomes necessary to increase the capacity of the transistor which forms the power source. Thus, it is impossible to decrease the capacity of the transistor by employing an over-current protection system which provides a particularly beneficial output voltage vs. load current characteristic for the power source.
(3) The load balance is insufficient (as described herein). Therefore, when the power sources switch such that a source which was previously non-conductive is rendered conductive, the output voltage drops significantly during the switch.
(4) Because of the characteristics of the matching diodes D1 and D2, the output voltage depend upon either the load current or the ambient temperature; that is, it is difficult to maintain the output voltage at a constant level with a high degree of accuracy.
(5) When the load current is high, the loss of electric power by the matching diodes D1 and D2 is high, and accordingly the efficiency is lowered.
FIG. 2 is a circuit diagram showing a second conventional power source system which is a so-called "master and slave system".
In FIG. 2, reference character M designates a master power source; S, a slave power source; Tr1 and Tr2, output voltage controlling transistors; A1 and A2, error amplifiers, ZD1, a Zener diode for supplying a reference voltage; and R14 and R24, output current detecting resistors.
The master power source M is an ordinary stabilized power source. In the master power source M, the conduction of the transistor Tr1 is controlled by the output of the error amplifier A1, such that a voltage applied to the inverting input terminal of the amplifier A1 (i.e., Vc=R13.multidot.EO/(R12+R13)) is equal to a voltage applied to the noninverting input terminal (i.e., reference voltage V.sub.ZD1), in order to maintain the output voltage EO constant.
On the other hand, in the slave power source S, the output of the error amplifier A2 is utilized to control the conduction of the transistor Tr2, so that a voltage applied to the noninverting input terminal of the amplifier A2 (i.e., the voltage at point b) is equal to a voltage applied to the inverting input terminal (i.e., the voltage at point a), which maintains the output voltage EO at a constant level. Therefore, when the voltage at point a is equal to that at point b, the following equation holds: EQU i2.multidot.R24=i1.multidot.R14 (1)
where, i1 is the current supplied to the load from the master power source M, and i2 is the current supplied to the load from the slave power source S.
If R24=R14, then i2=i1. That is, the current supplied to the load by the master power source M is equal to the current supplied to the load by the slave power source S. Accordingly, many of the drawbacks accompanying the diode matching system described with reference to FIG. 1 are substantially eliminated by this prior art master and slave system. However, the master and slave system is disadvantageous for the following reasons:
(1) When a slave power source is deactivated, it is "backed up" (i.e. current is supplied) by either the other slave power sources or the master power source. However, when the master power source is deactivated, its slave power sources are also deactivated. Thus, the reliability of the power source system in which the the power sources are operated in a parallel mode depends upon the master power source. Accordingly, in such a system, an increase in the number of slave power sources can increase the output capacity, but cannot improve the reliability of the system.
(2) The master power source is different in circuit arrangement from the slave power sources. When comparaed to the case where the master and slave power sources are equivalent in circuit arrangement, the master and slave system is not suitable for mass production. Accordingly, it is difficult to reduce the manufacturing costs and to decrease the time expended for the maintenance of such a parallel power source configuration.